1. Field of the Invention
The present invention relates to a liquid crystal display device including a thin film transistor at least in a pixel portion.
2. Description of the Related Art
In recent years, technology that is used to form a thin film transistor using a semiconductor thin film (with a thickness of from several nanometers to several hundreds of nanometers, approximately) formed over a substrate that has an insulating surface has been attracting attention. Thin film transistors are applied to a wide range of electronic devices like ICs and electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed.
For switching elements in image display devices, a thin film transistor using an amorphous semiconductor film, a thin film transistor using a polycrystalline semiconductor film, and the like are used. As a method for forming a polycrystalline semiconductor film, technology is known in which a pulsed excimer laser beam is processed into a linear shape by an optical system and used to scan and irradiate an amorphous semiconductor film for crystallizing the amorphous semiconductor film.
Also, as switching elements in image display devices, a thin film transistor using a microcrystalline semiconductor film is used (Reference 1: Japanese Published Patent Application No. H4-242724 and Reference 2: Japanese Published Patent Application No. 2005-49832).
A known conventional method for manufacturing the thin film transistor is that an amorphous silicon film is formed over a gate insulating film; a metal film is formed thereover; and the metal film is irradiated with a diode laser beam to modify the amorphous silicon film into a microcrystalline silicon film (Reference 3: Toshiaki Arai et al., SID 07 DIGEST, 2007, pp. 1370-1373). According to this method, the metal film formed over the amorphous silicon film is provided to convert optical energy of the diode laser beam into thermal energy and should be removed in a later step to complete a thin film transistor. That is, the method is that an amorphous semiconductor film is heated only by conduction heating from a metal film to form a microcrystalline semiconductor film.